The present invention relates to wood chippers and more particularly to wood chippers having a rotatable drum carrying knives angularly spaced about its circumferential wall.
Wood chippers have been developed to reduce trees, limbs, branches, and the like to wood chips. Such chippers are typically used by municipalities and tree services. Chipping eliminates environmental problems associated with burning; and the chips can be used in a wide variety of applications such as mulch and fuel. Basically, the chippers are of two broad types.
The first and oldest style of chipper is the "drumtype" wherein the chipper knives are carried on the circumferential wall of a cylindrical drum. Typically, the drum is approximately 12 inches in diameter and carries 4 equally spaced knives about its perimeter. The drum is driven at 2500 to 3500 revolutions per minute (rpm) so that the machines make approximately 10,000 to 14,000 cuts per minute. with an average chip thickness of 3/4 inch, the feed rate is 625 to 875 feet per minute. Such speeds are relatively fast and can be dangerous to operators feeding the chipper because, as the branches are pulled into the chipper, limbs can strike the workers. Further, the drum is typically a solid head with the knives clamped thereto. No space is provided for the chips. Accordingly, blowers or augers are included both to release the chips from the knives and to propel the chips through the chute. Engines with large horsepowers (i.e. in the range of 100 h.p. to 125 h.p.) are required to drive these chippers. Even with such engines, the drum chippers often stall. Further, the large engines require complicated and expensive emissions control equipment, especially in states such as California.
The second style of chipper is the "disc-type" wherein the knives are carried radially on the face of a spinning disc. An example of such a chipper is illustrated in U.S. Pat. No. 3,861,602 issued Jan. 21, 1975 to Smith and entitled Brush Chipper. The feed rates of disc-style chippers are significantly lower than those of drum-style chippers; and therefore the feed-speed problems are greatly alleviated. The distal end of the radially disposed knife travels at approximately 10,000 feet per minute, while the blade is traveling significantly slower towards the center of the disc. Consequently, chips removed at these different ends of the knife are travelling at different speeds and collide behind the disc and in the discharge chute to generally slow all of the chips down. Accordingly, the disc must be rotated faster than necessary to chip the wood to insure that the chips are properly discharged from the unit.